1. Field of the Invention
This invention relates to well logging methods and apparatus and, more particularly, to methods and apparatus for determining the porosity of the earth formations surrounding a borehole by irradiating the formations with neutrons and measuring the resulting spatial distribution of the neutrons within the formations. The subject matter of this invention is related to subject matter described in copending U.S. application Ser. No. 60.354 of Richard A. Bateman and Christian M. Clavier, filed of even date herewith and assigned to the same assignee.
2. Description of the Prior Art
A knowledge of the porosity of the formations surrounding a borehole is of fundamental importance in the oil industry. Porocity data is needed not only to identify possible oil or gas producing horizons, but also to calculate other important parameters, such as the maximum producible oil index of a specific formation.
Porosity measurements are generally performed by a dualdetector neutron porosity logging tool provided with a neutronemitting source that continuously irradiates the formation under study. The resulting neutron population is sampled by a pair of neutron detectors spaced at different distances from the source. The ratio of the counting rates from these detectors is computed and, with appropriate corrections, converted into a signal that corresponds to the formation porosity. A tool of this sort is described in more complete detail in U.S. Pat. No. 3,483,376 to S. Locke, patented Dec. 3, 1963.
Unfortunately, unless using a source many orders of magnitude stronger than the largest now in use, the detectors of a dual-detector neutron tool must be located at shorter spacings than would otherwise be chosen. As a consequence of the shorter source-detector spacings, the two registered counting rates are affected by porosity changes in slightly different ways. Hence, the porosity value derived from the ratio of these counting rates does not always reflect the true formation porosity.
Moreover, a dual-detector neutron tool is very sensitive to the presence of gas in the formations under investigation. If the tool is in a homogeneous bed of uniform gas saturation, both counting rates are increased because of the reduced hydrogen content of the formation. The porosity, as calculated from the ratio of these counting rates, is lower than the true porosity, but it can be corrected using the density reading of a dual-detector gamma radiation bulk density tool. A practical method for performing the correction for gas effect is described in "Computer Processed Interpretation of the rotliegendes Formation" June 1974 -- Well Evaluation Conference -- North Sea. However, superimposed on this effect in gas-bearing formations is the effect of the invasion profile. This means that the far detector may see a higher gas saturation than the near detector and thus read an even higher counting rate. The ratio of the counting rates, and hence the porosity derived from the ratio, drops even further. In this case, however, as the neutron tool has an appreciably deeper investigation than the density tool (this difference being a consequence of the specific spacings of each tool), the correction for gas effect is not performed in the best conditions of compatibility. As a consequence of this, accurate porosity measurements cannot always be obtained in invaded gas-bearing formations.
The presence of gas in an earth formation is often detected through a comparison of the signals acquired from a two-detector neutron porosity tool and a two-detector gamma radiation bulk density tool. The method is described in U.S. Pat. No. 3,566,117 to M. P. Tixier, patented Feb. 23, 1971. Due to the already mentioned lack of compatibility between the two compared readings, this method may not always lead to the best results. As well-known method for determining the density of a formation from the count rates of two detectors located at two different spacings from a common gamma radiation source, consists of converting said count rates into signals which are related to the formation density, comparing the converted signals to produce a correction signal representative of the difference therebetween, and adding the correction signal to one of the converted signals to produce a signal representing the true value of the formation density. The application of this method to the determination of the formation porosity from the two neutron population measurements produced by a dual-detector neutron porosity tool has been suggested in U.S. Pat. No. 3,701,902 to H. W. Janseen et al, patented Oct. 31, 1972. However, this patent does not teach how the neutron population measurements can be converted into porosity. Moreover, the above patent does not show how a dual-detector neutron tool can be used to detect gas-bearing formations.
Accordingly, it is an object off the present invention to provide an improved technique to evaluate the porosity of an earth formation.
It is another object of the invention to derive more accurate porosity measurements in invaded gas-bearing formations.
It is a further object of the invention to identify more precisely gas-bearing formations.
It is still a further object of the invention to provide an indication of the depth of invasion of a formation.